The present invention relates to tools for threaded fasteners generally, and more specifically to a method for applying a predetermined torque to a threaded fastener.
Threaded fasteners are commonly tightened with impact tools. An example of a field in which impact tools are used extensively is the automotive service market, in which impact tools are used for the reapplication of automotive wheels. Although impact tools are not designed to accurately control torque, many tire shops use impact tools as the primary means to re-apply lug nuts when mounting tires on automobiles. The current best practice in the industry includes re-applying the wheel lug nuts with an impact tool that has a torque stick attached to the output shaft and then hand tightening the nut 130 (see FIG. 1) with a hand torque wrench to verify torque. Torque sticks are designed to limit the maximum torque that an impact tool can apply to a nut 130, however, the actual torque achieved is determined by the impact wrench, air pressure, joint stiffness, and joint condition. Torque sticks only limit the torque applied; they do not allow the operator to specify a target torque, and there is no verification of the final joint torque. The two-step process of using an impact tool and then a torque wrench is also time consuming.
Tire shops have many different policies and procedures in place to attempt to improve quality, however, all the procedures rely heavily on the operator""s skill and consistency in performing the required steps. It is difficult for the tire shops to enforce their policies one hundred percent of the time, because a mechanic can complete the job using other available tools without following the proper procedure, and without applying the correct torque. Over or under tightening lug nuts can damage the wheel, hub and brake assembly. Damage to the wheel components can impact safety. Improperly tightened wheel lug nuts can potentially cause wheel separation.
Automobile manufactures publish very specific torque requirements for re-applying wheels to vehicles. Although tire shops may attempt to meet these specifications, their policies and procedures may not ensure detection of situations in which the lug nuts are tightened to an improper torque or not tightened at all. Several commercially available systems attempt to control the torque output of either an impact tool or a pulse tool.
The present invention provides a method of controlling an air driven tool to provide greater torque accuracy. The method comprises the steps of: establishing an air pressure profile for a plurality of torque values; determining a calibration factor for the tool; multiplying the desired torque by the calibration factor to determine a calibrated torque value; and supplying the tool with air at the air pressure profile corresponding to the calibrated torque value. The method may further include an improved technique for detecting previously tightened fasteners.